This proposal is to request support for a Keystone Symposia meeting entitled "Immunologic Memory and Host Defense", organized by Stephen P. Schoenberger and Susan M. Kaech, which will be held in Keystone, Colorado from February 8 - 13, 2009. Immune memory has a critical role in mediating protection against infections as well as potentiating certain allergic and autoimmune diseases. Hence a thorough understanding of the cellular and molecular mechanisms regulating adaptive immune memory will have important clinical application. In this regard, there has been great progress in understanding how the innate immune response influences adaptive immunity. Furthermore, improved methods to visualize immune responses in vivo, characterize the phenotypic and functional properties of adaptive immune responses and how lymphoid and non-lymphoid compartments influence the maintenance of such responses has substantially improved our understanding in this area. However, major hurdles still relate to difficulties in eliciting sustained T cell responses sufficient to mediate protection in humans with current vaccines. The goal of the meeting will be to focus on basic mechanisms for how T and B cells are programmed to induce and sustain immunity. The program is designed to integrate information from mouse, non-human primate and human studies to encompass all relevant areas related to control of memory T and B cell responses. In summary, this meeting should facilitate translational research that will impact vaccines and interventions for infectious disease, cancer, and autoimmune and allergic disease. This Keystone Symposia meeting will focus on the basic mechanisms of how T and B cells are programmed to generate and sustain immunity. Improved understanding of how immune responses are induced and maintained provides the fundamental basis for how vaccines work and will ultimately lead to more rational approaches to vaccine design.